466 INVEUTEBRATE MORPHOLOGY. 



rangement both of the corneal hypodermis and of the retinal elements, a 

 distinct raphe being observable upon the inner surface of the eye, the reti- 

 nal elements being arranged on either side of it. Such a condition as this 

 cannot readily be explained by a simple unilateral inversion such as was 

 described as probably occurring in Araohnidan eyes; it suggests rather an 

 inversion of two sides of a primitive optic cup, the posterior waU at the 

 same time forming the pigmented layer of the eye. Whether the Arachnid 

 eye is not also traceable to such an arrangement, all traces of the original 

 raphe being lost, is a question, though at present it seems more probable 

 that it has been produced by a suppression of the inversion of one side of 

 the cup. 



Glands, occurring in tlie palps and ovigerous legs, have 

 been regarded as excretory in function, but no Malpighian 

 tubules or coxal glands seem to exist, though an homology of 

 the excretory glands just mentioned and of glands occurring in 

 the walking-legs of the males with the latter is not impossible. 

 The Pycnogonids are bisexual, the reproductive organs lying 

 in the thorax and sending out branches into the walking-legs, 

 on the fourth joints of one or more of which they open. As 

 already stated, the male carries the eggs upon his ovigerous 

 legs, fastening them as they are extruded by the female by 

 means of the excretion of the glands occurring upon the 

 walking-legs. 



Development and Affinities of the Pycnogonida. — The young Pycno- 

 gonid leaves the egg as a six-limbed embryo, which recalls, in a general way, 

 the nauplius of the Crustacea, and indeed has suggested a derivation of 

 the Pycnogonids from that group. The resemblance is, however, but 

 superficial, important differences being found in the structure of the eyes 

 and in the absence of an anus, to say nothing concerning the details of the 

 early development. On the other hand these last, as well as the structure 

 of the eyes, recall the Arachnids, and it seems most probable that the 

 Pycnogonids are to be regarded as having descended from ancestors which 

 might have been included in the type Araohnida. 



Order Tardigrada. 



The Tardigrada are small forms not exceeding a milli- 

 metre in length, with an uusegmented body provided with 

 four pairs of short conical appendages tipped with claws, the 

 last pair being situated at the posterior extremity of the body. 

 The body is covered by a cuticle secreted by the subjacent 

 hypodermis, below which and traversing the coelom is a well- 



